Crude helium enrichment process

ABSTRACT

A crude helium stream is enriched by liquefaction and separation of contained impurities in a process wherein low temperature level refrigeration is provided by isentropic expansion of the enriched helium stream at its lowest temperature in the process. This invention resulted from work done by the Bureau of Mines of the Department of the Interior, and the domestic title to the invention is in the Government.

United States Patent Wayne W. Blackwell;

Herbert S. Kalman, both of Amarillo, Tex. 765,282

Oct. 7, 1968 Aug. 17, 1971 The United States of America as representedby the Secretary 01 the Interior Inventors Appl. No. Filed PatentedAssignee CRUDE HELIUM ENRICHMENT PROCESS 8 Claims, 1 Drawing Fig.

US. Cl 62/22, 23/2, 62/18, 62/23, 62/39, 62/26 Int. Cl F25j 1/02, F25 j3/06 Field of Search 62/11,22, 23, 24, 27, 28, 39; 23/2 References CitedUNITED STATES PATENTS 2/1959 Dennis OXYGEN CATALYTIC OXIDATION HELlUMPRODUCT Z3 2,932,173 4/1960 Mordhorst 62/22 3,181,307 5/1965Kuerstoni... 62/23 3,205,669 9/1965 Grossmann. 62/23 3,240,023 4/1966 DeLano 62/23 3,260,058 7/1966 62/23 3,293,869 12/1960 62/23 3,407,61410/1908 62/23 Primary lixaminvr- Norman Yudkoff Assistant Examiner-A. F.Purcell Attorneys-Ernest Si Cohen and Roland H. Shubert ABSTRACT: Acrude helium stream is enriched by liquefaction and separation ofcontained impurities in a process wherein low temperature levelrefrigeration is provided by isentropic expansion of the enriched heliumstream at its lowest temperature in the process.

This invention resulted from work done by the Bureau of Mines oftheDepartment ofthe lnterior, and the domestic title to the invention is inthe Governmentv DESICCANT N AND cm PATENTEDAUGI'IIHYI 3,599,43

OXYGEN DESICCANT CATALYTIC OXIDATION 3 HELIUM PRODUCT 2 3 V aeewcme V Q9N AND CH nvvnvrons WAYNE W BLACKWELL HERBERT 5. KALMA/V W $4, IBMH umATTORNEYS CRUDE HELIUM ENRICHMENT PROCESS BACKGROUND OF THE INVENTIONHeliumoccurs primarily as a minor constituent of natural gas and isfound principally in a few gas fields in the southwestern United States.Since helium is lost when heliumbearing natural gases are used for fuel,an extensive conservation program has been established to extract heliumfrom these natural gases before the gas is marketed.

Helium is extracted from helium-bearing natural gas by a low temperaturegas liquefaction process. The natural gas stream is cooled to atemperature below the liquefaction point of its ordinary constituentsbut above the liquefaction point of helium. A separation is made of thegas (helium) and the liquid (natural gas). The resulting produce of thisseparation is called crude helium and generally comprises about 70percent helium and 30 percent nitrogen. This crude helium is then eitherfurther purified to a marketable grade (99.995 percent pure) or isstored for future use.

Crude helium is stored underground in gas reservoirs until required foruse. Since few reservoirs ofa quality adequate for long term storage ofhelium without loss are available, it has become important to furtherenrich the crude helium so as to effectively increase the storagecapacity of the existing reservoirs. By enriching the crude helium from70 percent to 95 percent, for example, capacity of existing storagefacilities can be increased by more than one-third.

It has now been found that a crude helium stream may be substantiallyand economically enriched by a partial liquefaction process wherein lowtemperature level refrigeration to drive the process is provided byisentropic expansion of the enriched helium stream at its lowesttemperature level of the process.

It is an object of this invention to enrich a crude helium stream.

A further object of this invention is to remove impurities from a heliumstream without the requirement for an auxiliary low temperaturerefrigeration system.

DETAILED DESCRIPTION OF THE INVENTION The invention will be more clearlyunderstood from the following description of a preferred embodimentwherein reference is made to the accompanying drawing.

The FIGURE is a schematic'flow diagram ofa flow diagram of a preferredembodiment of the process.

Referring now to the FIGURE, a relatively high pressure crude heliumstream 1 is passed through preheater 2 and thence into reactor 3. Stream1 generally contains about 50 to 80 mol. percent helium with the balancebeing mostly nitrogen. Hydrogen, methane, neon and argon are alsogenerally present in small quantities. Sufficient air or oxygen tocompletely react with the hydrogen present in stream 1 is introducedinto the reactor via line 4. In reactor 3, hydrogen is removed bycatalytic oxidation in a manner well known in the art.

Gas is removed from reactor 3 via line 5, is cooled in heat exchanger 6and is then passed to separator 7 where any liquid water formed by thehydrogen oxidation is removed through conduit 8. From the separator, thegas is then passed through line 9.to drier l0. Drier 10 comprises a bedof desiccant such as activated alumina, silica gel or molecular sieves.Preferably a plurality of drier units are employed in order to allowregeneration of the desiccant without interruption of stream flow.

The dried gas is then passed, via line 11, through heat exchanger 12where it is cooled by heat exchange with cooler gas flowingcountercurrent therewith. Operation of this heat exchanger will beexplained in greater detail later. A cooled and partially liquifiedcrude helium stream is removed from heat exchanger 12 and is passed byway of conduit 13 to the first stage ofa multiflash separator 14 whereinliquid, comprising mostly nitrogen and methane. is separated from heliumgas. Liquid product from the first stage is removed and passed throughexpansion valve 15 to the second stage of separator 14 which ismaintained at a relatively low pressure; generally on the order of 20 to50 p.s.i.a. Dissolved helium present in the liquid from the first stageflashes under the reduced pressure in the second stage and is removedfrom separator M by means of line 16. This cold gaseous helium stream ispassed through heat exchanger 12 wherein it is warmed by heat exchangewith the crude helium feed stream and then to recycle compressor 17where it is compressed to a pressure somewhat exceeding that of the feedstream. Heat of compression is removed in cooler 18 and the recyclestream is then merged with the crude feed in line 11.

Liquid from the second stage of separator 14, comprising mostly nitrogenwith only trace amounts of helium, is passed by line 19 through heatexchanger 12 where it vaporizes and is warmed by indirect countercurrentheat exchange with the crude stream. This warm stream is then removedfrom the system by way of conduit 20.

Gas comprising enriched helium at a pressure substantially equal to thatof the crude feed stream is removed from the first stage of separator 14by way of conduit 21 and is then isentropically expanded to a pressuresomewhat below that of the crude feed. Since the helium stream leavingseparator 14 will not cool when it is throttled, refrigeration cannot beobtained by JouleThomson expansion. Isentropic expansion is performedusing an expansion engine 22, which may be either a reciprocating pistonor turbine device, in spite of the fact that the gas is at its dew pointand liquid forms during the expansion. By limiting liquid formation toless than about 2 percent and preferably to less than about 1 percent,adequate low temperature level refrigeration is obtained to drive theprocess without interfering with expansion engine operation. Expandedand cooled gas containing a small quantity of liquid is passed via line23 to heat exchanger 12 where it is warmed by countercurrent heatexchange with the incoming crude stream. This product stream then passesfrom the system via line 24 for storage, further processing orutilization.

Application of the process to a particular feed stream is shown in theTable. In each case, the stream designation refers to thecorrespondingly numbered location of the flow diagram.

At a feed rate of 18 MMscfd, the recycle compressor 17 requires about125 BHP, while about 23 BHP is produced by expansion turbine 22.Previous designs for helium enrichment process employed an auxiliarynitrogen refrigeration system to provide low temperature levelrefrigeration. Power requirements for the auxiliary refrigeration systemalone at a feed rate of 18 MMscfd are on the order of 600 BHP.

Crude helium streams having a helium content of about 50 to mol. percentare satisfactory as a process feed stream. Product purity will rangefrom about to 98 mol. percent helium depending upon the feed streamconcentration and processing conditions used. Pressure of the crudestream is desirably in the range of about 500 to 2,500 psi. The productgas obtained is sufficiently pure for some industrial uses but, in theexample illustrated, enrichment was for the purpose of increasing thecapacity ofa limited storage area.

What is claimed is:

I. A cryogenic method for enriching a crude helium stream containing atleast about 50 mol. percent helium which comprises:

a. passing the crude stream in indirect countercurrent heat exchangerelationship with colder process streams to at least partially liquifythe gaseous impurities curtained in the crude helium stream;

b. separating liquid impurities from gaseous helium in a firstvaponliquid separation zone maintained at a pressure substantially equalto that of the crude stream;

c. removing an enriched gaseous helium stream containing about 90 to 98mol. percent helium from the first zone, said gaseous stream comprisinga major portion of the crude helium feed stream and being at its dewpoint, and isentropically expanding the stream to obtain refrigerationto drive the process, to further cool the stream and to liquify a minorportion thereof, and

d. passing the cooled partially liquified enriched helium stream derivedfrom the isentropic expansion in countercurrent, indirect heat exchangerelationship with the incoming crude stream.

2. The process of claim 1 wherein separated liquid is removed from thefirst zone and is passed to a second vaporliquid separation zonemaintained at a pressure substantially less than that of the first zoneso as to flash dissolved helium from the liquid impurities.

3. The process of claim 2 wherein a vapor stream comprising helium isremoved from the second zone. is passed in countercurrent heat exchangerelationship with the incoming crude stream and is thereafter compressedand merged with the incoming crude stream.

4. The process of claim 3 wherein helium-depleted liquid is removed fromthe second separation zone and is passed in indirect heat exchangerelationship with the incoming crude stream.

5. The process of claim 4 wherein said isentropic expansion is performedin an expansion engine and wherein expanded gas is thereafter passed inindirect, countercurrent heat exchange with the incoming crude stream.

6. The process of claim 5 wherein liquid formation in the expansionengine is limited to less than about 2 mol. percent of the gas streambeing expanded.

7. The process of claim 6 wherein hydrogen contained in the crude heliumstream is removed by catalytic reaction with an oxygen-containing gasprior to heat exchange with colder process streams.

8. The process of claim 6 wherein the crude helium stream contains about50 to mol. percent helium and wherein the isentropically expanded heliumstream contains about to 98 mol. percent helium. w

2. The process of claim 1 wherein separated liquid is removed from the first zone and is passed to a second vapor-liquid separation zone maintained at a pressure substantially less than that of the first zone so as to flash dissolved helium from the liquid impurities.
 3. The process of claim 2 wherein a vapor stream comprising helium is removed from the second zone, is passed in countercurrent heat exchange relationship with the incoming crude stream and is thereafter compressed and merged with the incoming crude stream.
 4. The process of claim 3 wherein helium-depleted liquid is removed from the second separation zone and is passed in indirect heat exchange relationship with the incoming crude stream.
 5. The process of claim 4 wherein said isentropic expansion is performed in an expansion engine and wherein expanded gas is thereafter passed in indirect, countercurrent heat exchange with the incoming crude stream.
 6. The process of claim 5 wherein liquid formation in the expansion engine is limited to less than about 2 mol. percent of the gas stream being expanded.
 7. The process of claim 6 wherein hydrogen contained in the crude helium stream is removed by catalytic reaction with an oxygen-containing gas prior to heat exchange with colder process streams.
 8. The process of claim 6 wherein the crude helium stream contains about 50 to 80 mol. percent helium and wherein the isentropically expanded helium stream contains about 90 to 98 mol. percent helium. 